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  <title>Document</title>
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<body>
  <!-- 描述
    请补全JavaScript代码，要求以字符串的形式返回文件名扩展名，文件名参数为"filename"。
  -->
  <script>
    const _getExFilename = (filename) => {
      // 补全代码
      var fileFix = filename.split('.')
      if (fileFix.length < 2) {
        return 'unkown'
      }
      return `.${fileFix[fileFix.length - 1]}`
    }
  </script>
</body>
</html>